1. Field of the Invention
The present invention relates to continuous wave radar ranging systems and, in particular, to a signal processor for use therein.
2. Description of the Prior Art
In continuous wave (CW) radar ranging systems, a frequency modulated interrogation signal is transmitted toward a target and is reflected therefrom back to the interrogating unit. The reflected signal is received by the interrogating unit, mixed with a sample of the interrogation signal, and filtered to obtain a difference signal. The finite distance or range between the interrogating unit and the target introduces a round trip delay .tau. between the return signal and the instantaneous interrogation signal sample. Expressed mathematically, EQU .tau.=2R/C (1)
where R is the range and C is the velocity of light. The interrogation signal is frequency modulated with a given modulation waveform; and the reflected signal as received at the interrogating unit is delayed in time, and hence shifted in frequency, from the instantaneous interrogation signal by an amount proportional to the range. For example, where a triangular waveform having a total frequency excursion of .DELTA.F and a period of 1/f.sub.m is used to frequency modulate the interrogation signal, the frequency shift or difference frequency f.sub.R, as generated by a suitably filtered mixer, equal to the time derivative of the frequency of the interrogation signal times the round trip time delay, is: EQU f.sub.R =.tau.(df/dt)=4(.DELTA.F)f.sub.m R/C (2)
Thus, the range between the target and the interrogating station may be computed by a measurement of frequency shift f.sub.R.
Conventional processors measure the difference frequency by counting the number of zero crossings in the difference signal that occur within a fixed time interval. More specifically, the difference signal is applied to a counting circuit which develops a signal that is proportional to the rate of zero crossings.
However, the difference signal waveform undergoes periodic phase discontinuities at a rate of twice the frequency (f.sub.m) of the modulation waveform of radar 10. For a description of such phenomenon reference is made to U.S. Pat. No. 3,968,492, issued July 6, 1976, to G. S. Kaplan and to U.S. Pat. No. 3,974,501, issued Aug. 10, 1976, to A. B. Ritzie, both assigned to the assignee of the present invention. It should be appreciated that the phase discontinuities cause the number of zero crossings occurring during a half cycle of the FM waveform to vary, resulting in an ambiguity in the indicated range. Such ambiguity is particularly evident in that a target receding from radar 10 by a distance equal to one quarter (1/4) wavelength of the transmitted signal frequency may, due to the variation in the number of zero crossings during the half FM waveform cycle, appear to advance toward the target.
This phenomenon can be an acute problem in systems operating on targets having complex or changing reflective surfaces. For a more detailed description of such phenomenon, reference is made to "Frequency Modulated Radar," D. G. C. Luck, Chapter 4, McGraw-Hill, 1949.
For a description of various methods to minimize quantization of the measured difference frequency due to the phase discontinuities reference is made to the above cited Kaplan and Ritzie patents.
There are, however, numerous applications in which during the desired range measuring period, the range increases or decreases monotonically, that is, without reversing directions. An example of such monotonic ranging is the measurement of the level of a material in a container as the container is being emptied. Such a measurement is often made in the operation of an iron blast furnace. The furnace is filled with "burden", a mixture of iron ore, coke, and limestone, and as the burden is melted to form molten iron, the level of the burden in the furnace decreases. When the level of the burden in the furnace decreases to a predetermined level, new burden is added through a gate located in the top of the furnace. Molten iron and slag are removed every 15 to 30 minutes from the bottom of the furnace.
Another such application is the monitoring of the drilling rate and depth of a drilling rig bit where during the drilling process, the drill bit continually penetrates deeper into a drilled material.
U.S. Pat. No. 4,072,947, issued Feb. 7, 1978, to H. C. Johnson and assigned to the assignee of the present invention, describes a simplified signal processor for reducing quantization error in such a monotonic FM-CW radar ranging system. Included in this processor is a first phase-locked loop (PLL) including a voltage-controlled oscillator (VCO), a phase comparator for comparing the average-axis or "zero" crossings of the oscillations with those of the frequency shift or difference frequency f.sub.R, and a low-pass filter coupling the comparator output to the control circuit of the VCO to complete the phase-locked loop. The first PLL is a frequency-tracking filter. Choosing the cut-off frequency of the low-pass filter to be appreciably lower in frequency terms than the rate at which the radar CW is frequency-modulated allows the output frequency on average to track f.sub.R, but will prevent its phase from exhibiting the phase discontinuities associated with f.sub.R. While this method of eliminating phase discontinuities works reasonably well in practice, I have found that there are some aspects in which performance could be improved. In these systems a common antenna is used both for transmission of the radar CW and for reception of its reflection from target, a circulator being used to implement this. As the CW being transmitted sweeps through certain frequencies, the distance to target will be expressed in odd multiples of quarter wavelength; and the carrier wave will not propagate to be received as a strong reflection, i.e., there is a "phase cancellation" of received signal leading to the difference frequency (f.sub.R) output from the mixer essentially disappearing. The mixer output is coupled to the first PLL by a chain of amplifiers and filters. This chain ends in wide-band amplifiers which are driven into symmetrical limiting on peaks of f.sub.R signal to suppress noise by virtue of the capture phenomenon, except when phase cancellations occur. When f.sub.R disappears, the residual noise in the output signal from this chain pulls the PLL towards higher frequency, causing a so-called, "run-up" condition in which the range measurement errs towards being overlong. Phase cancellations of this severity--so-called "major" phase cancellations--occur only if axis of the radar beam is perpendicular to the surface of the target. Less severe "minor" phase cancellations occur where the surface is rough or does not fall in a plane to which the axis of the radar beam is perpendicular. During these minor phase cancellations the system noise is not well suppressed, and the first PLL is undesirably pulled in frequency by it.